Freebsd support

@sethvargo @schisamo 

FreeBSD support for the omnibus cookbook.

Running ruby-install via Mixlib::ShellOut is a hack, and I am not attached to it.  On FreeBSD, ruby-install within the original execute block in the ruby_install provider fails silently and quickly, and the real error seems to be swallowed; I couldn't reproduce this using any other method of running ruby-install either manually or from chef.